1. Field of the Invention
The present invention relates to a displacement detection apparatus which detects a displacement by detecting a light diffracted by a diffraction grating, a displacement measurement apparatus which measures the amount of displacement using an interference of the lights diffracted by diffraction gratings, and a fixed point detection apparatus which acquires a fixed point by means of the diffracted lights.
2. Description of the Related Art
In the past, there has been a grating interferometer as a detector which detects a displacement of the position of a diffraction grating attached onto a moving scale by using the interference of lights. In the following, a displacement detection apparatus is described with reference to FIG. 18. It is noted that FIG. 18 shows a displacement detection apparatus using a transmission grating.
As shown in FIG. 18, the displacement detection apparatus is equipped with a coherent light source unit 90, a first lens 91, a first polarization beam splitter (PBS) 92, a first quarter wavelength plate 93, a reflecting prism 94, a second quarter wavelength plate 95, a second lens 96, a beam splitter (BS) 97, a second PBS 98, a first photoelectric transducer 99, a second photoelectric transducer 100, a third quarter wavelength plate 101, a third PBS 102, a third photoelectric transducer 103, a fourth photoelectric transducer 104, a first differential amplifier 105, a second differential amplifier 106 and an incremental signal generator 107. The displacement detection apparatus reads a transmission grating disposed on a scale 108.
The coherent light source unit 90 emits a light to the first lens 91. The first lens 91 narrows down the entered light into a suitable beam, and emits the beam to the first PBS 92. The first PBS 92 divides the entered light into two lights, a light having an S polarization component and a light having a P polarization component. The S polarization component is a polarization component oscillates perpendicular to an incident plane composed of a light entering a boundary surface of lights and a light reflected by the boundary surface. Moreover, the P polarization component is a polarization component performing vibrations horizontal to the incident plane. The light having the S polarization component is reflected by the first PBS 92, and the light which has the P polarization component is transmitted through the first PBS 92. It is noted that, if the light from the coherent light source unit 90 is the linearly polarized light, the light is entered into the first PBS 92 with the polarization direction thereof inclined by 45 degrees. Herewith, the intensity of the light of the S polarization component and the intensity of the light of the P polarization component can be made equal to each other.
The light having the S polarization component, which has reflected by the first PBS 92, enters a point P of the diffraction grating recorded on the scale 108. Moreover, the light having the P polarization component, which has transmitted through the first PBS 92, enters a point Q of the diffraction grating. Each of the lights is diffracted into the direction expressed by the following formula.sin θ1+sin θ2=n·λ/Λwhere θ1 denotes an incident angle onto the scale 108; θ2 denotes the angle of diffraction from the scale 108; Λ denotes the pitch (width) of the lattice; λ denotes the wavelength of the light; and n denotes a diffraction order.
If the following marks are supposed to each of the following factors: θ1p to an incident angle into the point P; θ2p to the angle of the diffraction at the point P; θ1q to an incident angle into the point Q; and θ2q to the angle of the diffraction at the point Q, then the related art displacement detection apparatus shown in FIG. 18 adjusts θ1p, θ2p, θ1q and θ2q so as to be θ1p=θ2p=θ1q =θ2q. Moreover, the diffraction orders at the points P and Q are the same orders.
The light (S polarization component) having been diffracted at the point P passes through the first quarter wavelength plate 93, and is perpendicularly reflected by the reflecting prism 94. Then, the reflected light again returns to the point P to be diffracted by the diffraction grating. Because the optical axis of the first quarter wavelength plate 93 is inclined to the polarization direction of the entered light by 45 degrees at this time, the light having returned to the point P is changed to the light of the P polarization component.
Moreover, the light (P polarization component) having been diffracted at the point Q similarly passes through the second quarter wavelength plate 95, and is perpendicularly reflected-by the reflecting prism 94. Then, the reflected light again returns to the point Q to be diffracted by the diffraction grating. Because the optical axis of the second quarter wavelength plate 95 is inclined to the polarization direction of the entered light by 45 degrees at this time, the light having returned to the point Q has been changed to the light of the S polarization component.
The lights diffracted again at the points P and Q in this manner return to the first PBS 92. Because the light which has returned from the point P has the P polarization component, the light passes through the first PBS 92. Moreover, because the light which has returned from the point Q has the S polarization component, the light is reflected by the first PBS 92. Consequently, the lights which have returned from the points P and Q are superposed by the first PBS 92, and are narrowed down to a suitable beam by the second lens 96 to enter the BS 97.
The BS 97 divides the entered light into two lights. Then, the BS 97 enters one light into the second PBS 98, and enters the other light into the third quarter wavelength plate 101. Note that the second PBS 98 and the third quarter wavelength plate 101 are severally inclined to the polarization direction of the entering light by 45 degrees.
The light having entered the second PBS 98 is divided into a light having the S polarization component and a light having the P polarization component. The light having the S polarization component is entered into the first photoelectric transducer 99, and the light having the P polarization component is entered into the second photoelectric transducer 100. Moreover, an interference signal of Acos(4Kx+δ) is acquired in the first photoelectric transducer 99 and the second photoelectric transducer 100, where K denotes 2π/Λ; x denotes a movement quantity; and δ denotes an initial phase. Moreover, in the first photoelectric transducer 99, a signal having a phase different from that in the second photoelectric transducer 100 by 180 degrees is acquired.
Moreover, a light which has entered the third quarter wavelength plate 101 is changed into a light having the P polarization component and a light having the S polarization component, both being mutually reversed to circularly polarized lights, and then the circularly polarized lights are superposed on each other to become a linearly polarized light. Then, the linearly polarized light enters the third PBS 102. The light having entered the third PBS 102 is divided into a light having the S polarization component and a light having the P polarization component. The light having the S polarization component is entered into the third photoelectric transducer 103, and the light having the P polarization component is entered into the fourth photoelectric transducer 104. It is noted that the polarization direction of the linearly polarized light entering the third PBS 102 makes one revolution when the diffraction grating moves into the x direction by Λ/2. Consequently, the third photoelectric transducer 103 and the fourth photoelectric transducer 104 can acquire an interference signal of Acos(4Kx+δ′) in common with the first photoelectric transducer 99 and the second photoelectric transducer 100. Moreover, in the third photoelectric transducer 103, a signal of a phase different from that of the fourth photoelectric transducer 104 by 180 degrees is acquired.
In addition, the third PBS 102 is inclined to the second PBS 98 by 45 degrees. Consequently, the signals acquired by the third photoelectric transducer 103 and the fourth photoelectric transducer 104 differ from the signals acquired by the first photoelectric transducer 99 and the second photoelectric transducer 100 in phase by 90 degrees.
The first differential amplifier 105 performs the differential amplification of the electric signals input from the first photoelectric transducer 99 and the second photoelectric transducer 100, and outputs a signal acquired by cancelling the direct current (DC) component of the interference signal to the incremental signal generator 107. Moreover, also the second differential amplifier 106 similarly performs the differential amplification of the electric signals input from the third photoelectric transducer 103 and the fourth photoelectric transducer 104, and outputs a signal acquired by cancelling the direct current (DC) component of the interference signal to the incremental signal generator 107.
Next, FIG. 19 shows an example of a related art fixed point detection apparatus disclosed in Published Unexamined Japanese Patent Application No. Hei 4-324316, filed by the present applicant. The fixed point detection apparatus includes a fixed unit 110 and a movable unit 130 movable along a measurement direction (X direction). The fixed unit 110 includes an optical system 111 and a detection system 121, and the movable unit 130 includes a substrate 131 and two volume type holographic diffraction gratings 132 and 133 disposed on the top face of the substrate 131.
The optical system 111 includes a light source 112 such as a semiconductor laser or the like, which outputs a laser light, a collimator lens 113 and a condenser lens 114. The detection system 121 includes two photo-receivers 122 and 123 and electric processing circuit 129.
The holographic diffraction gratings 132 and 133 used for this example are shown in FIG. 20. The holographic diffraction gratings 132 and 133 are severally formed of a volume type hologram of a transmission type. In the following, the holographic diffraction gratings 132 and 133 are simply called as holograms on occasion. As shown in FIG. 20, the grating interval, or a grating pitch d, of each of the holograms 132 and 133 sequentially and continuously changes in the measurement direction. Moreover, distribution surfaces 142 and 143, on which the grating interval or the grating pitch d, of each of the holograms 132 and 133 is severally formed, are inclined to the top faces of the holograms 132 and 133, and the angles of the inclinations sequentially and continuously change in the measurement direction. If an incidence light is diffracted by each of the holograms 132 and 133, the diffraction efficiency of the incident light continuously changes in the measurement direction.
FIG. 21 shows the principal part of the fixed point detection apparatus of FIG. 20. As shown in FIG. 21, the two holograms 132 and 133 are mutually disposed to adjoin in a lateral direction on the top face 131A of the substrate 131. These two holograms 132 and 133 are mutually configured symmetrically to the center plane 135. That is, the angles of the inclinations of the distribution surfaces 142 and 143 of each of the holograms 132 and 133 sequentially and continuously change on both sides symmetrically to the center plane 135, and the grating intervals, or the grating pitches d, sequentially and continuously change to both sides symmetrically to the center plane 135. The two holograms 132 and 133 are disposed so that the points at which the diffraction efficiency of each of the holograms 132 and 133 becomes the maximum may be different from each other in the measurement direction.
When the movable unit 130 moves relatively to the fixed unit 110, namely when the movable unit 130 moves to the photo-receivers 122 and 123 and the light source 112, which are at rest, in FIG. 21, the light diffracted by the first hologram 132 is detected by the first photo-receiver 122, and the light diffracted by the second hologram 133 is detected by the second photo-receiver 123.
Because the points at which the diffraction efficiencies of the two holograms 132 and 133 severally become the maximums are different from each other, the peak position of the light intensity curve of the diffracted light detected by the first photo-receiver 122 and the peak position of the light intensity curve of the diffracted light detected by the-second photo-receiver 123 are different from each other. Consequently, there is an intersecting point of the two light intensity curves, namely a point where two light intensities become equal to each other. This point is a fixed point acquired by the fixed point detection apparatus.